Light guiding in optical waveguides, and light guiding in optical fibres in particular, is a well-known technology for transporting energy and information in the form of light. For example, as in the case of optical fibres, one-dimensional optical waveguides are based on light guiding in a medium of cylindrical symmetry. The light guiding takes place in a core, which is surrounded by a medium having a lower refractive index, the so-called cladding, light guiding according to a simple model being obtained by means of repeated total internal reflections between the core and the cladding. However, the light can only propagate in certain predetermined directions, so-called modes, which are defined by certain phase conditions which must be met in connection with the propagation of the light. According to the standard model, these modes consist of eigensolutions to Maxwell's equations applying existing cylindrical boundary conditions.
If the cross-sectional dimension of the core is sufficiently small, the light can only propagate in a single such mode. An optical waveguide with this characteristic is called an optical monomode waveguide. Monomode waveguides have certain important advantages over a waveguide permitting several modes (multimode waveguide). For example, the information transfer capacity of an optical monomode fibre, often called an optical single-mode fibre, is much greater than that of a multimode fibre when light is guided through a long fibre. Another important advantage of a monomode waveguide such as a single-mode fibre is its lack of ambiguity. Apart from the polarisation state of the light, the characteristics of the light will be well-defined along the entire waveguide. In particular, the intensity distribution of the light will be well-defined along the entire waveguide. This is extremely important in order to provide predictable operation of waveguide-based components.
Generally, several separate channels are utilised in order to increase the transfer capacity of an optical waveguide, each channel consisting of a specific light, wavelength. This technology is called wavelength division multiplexing or WDM. In connection with WDM it is thus desirable to be able to add and subtract single wavelength channels, i.e. single light wavelengths, to and from the waveguide.
A well-known technology for wavelength-selective alteration of the propagation direction of light utilises optical phase gratings. An optical phase grating is a structure of essentially periodically varying refractive index in an optically transparent medium. When light is incident upon an optical phase grating a small part of the incident light is reflected by each grating element (period). When a plurality of grating elements are arranged in succession (i.e. arranged in a phase grating) the total amount of reflected light will be the sum of all of these separate reflections. The part of the incident light that is reflected by each grating element depends on the depth (amplitude) of the refractive index modulation of the phase grating, i.e. on the refractive index difference of the grating elements. The greater the modulation the greater the part of the incident light that is reflected by each phase element. If the propagation direction of the light which is incident upon a phase grating is essentially perpendicular to the grating, i.e. to the normal of the grating elements, the grating is said to be operating in the Bragg domain and is called a Bragg grating. As a result of the perpendicular incidence the light will be reflected essentially parallel to the direction of incidence (i.e. in the opposite propagation direction). The light which is reflected by each grating element will thus overlap the light reflected by all the other grating elements, thus giving rise to interference. In a monomode waveguide, all reflections within a certain angle cone will couple to the only mode (propagation direction) permitted by the waveguide. In the case of the wavelength where these reflections are in phase, constructive interference arises, and despite the fact that each grating element only provides a low intensity reflection, substantial reflection will be obtained for this wavelength from the grating as a whole. This wavelength, at which a substantial reflection is obtained from the grating as a whole, is called the Bragg wavelength λbragg and is given (in connection with perpendicular incidence) byλbragg=2nΛwhere n is the average value of the refractive index and Λ is the period of the phase grating. The reflectance for the Bragg wavelength is given byRbragg=tan h2 κL where L is the length of the Bragg grating in the propagation direction of the light and κ is defined as
  κ  =            4      ⁢                          ⁢      π      ⁢                          ⁢      Δ      ⁢                          ⁢      n        λ  where Δn is the amplitude of the refractive index modulation. Since the refractive index modulation Δn typically is small (10−5–10−3), the above expression of the reflectance can be expanded into a power series, whereby it can be seen that the reflectance is approximately proportional to the square of Δn.
If the angle of incidence of the light upon the phase grating is not perpendicular, i.e. if the grating planes are inclined, the light will not be reflected in the direction of incidence. By utilising an inclined phase grating, also known as a blazed grating, light can be coupled out from the core of the waveguide. Similarly, light can be coupled into the core of the waveguide by means of a blazed grating.
U.S. Pat. No. 5,042,897 (Meltz et al.) describes a device for coupling light from a waveguide with the aid of tilted (blazed) gratings, i.e. phase gratings having grating elements (refractive index variations) whose planes intersect the propagation axis of the waveguide under an angle which is different from 90 degrees. The angle at which the light will be coupled from the waveguide is determined by the angle of inclination of the grating elements in relation to the propagation axis of the waveguide (the transverse phase matching condition) as well as by the wavelength (the longitudinal phase matching condition). The tilted grating elements function as small, almost completely transparent, mirrors. The diameter of the mirrors (grating elements) is essentially equal to the diameter of the waveguiding structure. In a single-mode fibre, for example, the waveguiding structure is composed of the core of the fibre, which usually has a diameter of about 10 micrometers. Since this diameter is not much greater than the wavelength of the light, the mirrors (grating elements) will cause diffraction of the reflected light. Consequently, the reflected light will spread out in a cone around the angle defined by the angle of inclination of the grating elements. The transverse phase matching condition gives that this angle is about twice as large as the angle of inclination. Since the grating elements reflect light which is partially overlapping, a certain wavelength will only give rise to constructive interference if the light from each consecutive grating element is in phase with the light from the preceding grating element. This occurs at a certain predetermined angle, which is given (for a longitudinal grating) by the longitudinal phase matching condition
                    2        ⁢                                  ⁢        π        ⁢                                  ⁢                  N          eff                    λ        +                            2          ⁢                                          ⁢          π          ⁢                                          ⁢                      n            clad                          λ            ⁢      cos      ⁢                          ⁢              φ        L              =                    2        ⁢                                  ⁢        π            Λ        ⁢    cos    ⁢                  ⁢          θ      g      where Neff and nclad are the refractive indices of the waveguiding structure (core) and the substrate (cladding) respectively, the substrate being assumed, in the above expression, to have an infinite extension, φL being the output-coupling angle in the cladding, and θg being the angle of inclination.
Hence, light coupled out from a waveguiding core by means of blazed gratings will exhibit significant divergence. Obviously, this divergence causes problems when the light is to be further processed. For example, it might be desirable to detect the light, modulate the light or direct the light into another waveguiding core. Consequently, improvements are needed in order to overcome this problem of divergence.
It is to be understood that light beams, as described in this application, are to be regarded as Gaussian beams. For example, this means that beams exhibit a waist when focused, and are diffraction limited as regards divergence. The characteristic features of Gaussian beams are well k)own to the man skilled in the art.